Generally, a gas having high chemical reactivity is supplied to the process chamber used in the semiconductor manufacturing facilities and the like. Accordingly, the vacuum exhaustion system for the process chamber is required to exhaust high reactivity gases in safety and with a high degree of efficiency.
Normally, the piping system in the semiconductor manufacturing facilities comprises a system to supply a gas to the process chamber, the process chamber, the vacuum exhaustion system, vacuum pumps, a diaphragm valve and the like. For the vacuum pumps, a plural number of pumps, that is, a primary pump (of the high vacuum type) installed immediately after the process chamber and a secondary pump (of the low vacuum type) installed on the secondary side thereof are employed. A turbocharged molecular pump is used for the high vacuum type one, while a scroll type pump is used for the low vacuum type one.
To effect an efficient exhaustion from the process chamber, it is necessary to employ an exhaustion pump having a high compression ratio which can operate with a high velocity of exhaustion (1/min) even if the suction pressure is low. However, in reality, the vacuum exhaustion pump having a high compression ratio is not easily available. Therefore, in the conventional system for the vacuum exhaustion from the process chamber, in order to deal with two challenges, i.e., one that the gas is to be exhausted with a high degree of efficiency using a pump having a relatively low compression ratio, and the other that the pump overload is to be avoided by keeping small a pressure difference between the primary side and secondary side of the vacuum exhaustion system, the pipings having a large diameter (a nominal diameter of approximately 4 inches) have been employed. For the same reason, the diaphragm valve with a large diameter has been employed.
The fluid flow is classified into two regions, i.e., the viscous flow region and the molecular flow region with regard to the relationship between the pressure and the inside diameter of the flow passage. To effect an efficient exhaustion, it is required that the exhaustion be conducted in the viscous flow region. To achieve the viscous flow region, the inside diameter D of the flow passage should be L≦D (where L: the mean free path of the gas molecule and D: the inside diameter of the flow passage). There exists the relation, L=4.98×10−3 /P, between the mean free path of the gas molecule L and the pressure P. On the basis of this, the relationship between the pressure and the inside diameter to attain the viscous flow region inside the pipings is obtained. Accordingly, by raising the pressure higher, the mean free path L can be made smaller with the result that the inside diameter D of the pipings to attain the viscous flow region is made small.
However, as stated above, with the conventional pump having a comparatively small compression ratio (approximately 10), it is not possible to raise the pressure on the discharge outlet side. For example, if the pressure on the chamber side (the intake inlet side) is 10−3 Torr, the discharge outlet side pressure becomes as low as approximately 10−2 Torr. This means that the pipings having an inside diameter of 5 cm or more are required to attain the viscous flow region with more certainty. As a result, with the conventional vacuum exhaustion system, there is a problem that since the piping system having a large diameter is required, the facilities are made large-sized. Further, there is another problem that since a larger inside diameter of the vacuum pipe system means a larger volume inside the pipe, it takes a longer time for the vacuum exhaustion Furthermore, to effect the exhaustion operation efficiently in a short time by the vacuum exhaustion system, an expensive vacuum pump having a large compression ratio and a high velocity of exhaustion is needed.
In recent years, however, the pump having high performance capabilities, or specifically, one which realizes a high compression ratio of approximately 103˜104 has been developed. As a result, it is now possible to increase the discharge side pressure of the primary pump to approximately 30˜50 Torr even when the internal pressure of the process chamber is around 10−3 Torr. Accordingly, by optimizing the pressure conditions of the process chamber and the vacuum exhaustion system, it is now possible to secure the viscous flow region in the pipings having a small diameter of approximately 0.5 cm.
However, if the pressure is raised as mentioned above, moisture or gas condenses in the vacuum exhaustion piping system and adheres to the inside of the pipings.
Further, even if the condensation and adherence of water, moisture or gas caused by the pressure rise do not occur, the decomposition of the gas remaining inside the pipings happens when the vacuum pump is out of operation. As a result, substances produced by the gas decomposition accumulates inside the pipings and piping parts of the valve, causing the corrosion of the parts, the clogging of the valves by the substances as produced and the valve seat leakages.
To solve such problems, it is required to keep the inside of the piping system lower than the saturated vapor pressure of gas, water or moisture therein. Hence, it is a common practice, with the vacuum exhaustion system, to effect heating (baking). (In the case of water, the saturated vapor pressure is 17.53 Torr at 20° C.) That is, when the temperature is raised by the heating, the saturated vapor pressure rises, making it difficult for the condensation and adherence of water, moisture or gas to occur with the result that the risk of the corrosion and cloggings is reduced. It is known that it is desirable to raise the temperature to approximately 150° C., considering the type of the gas inside the vacuum exhaustion system.
However, the gas is decomposed (or dissociated) when the temperature rises. Here occurs another problem that the substances produced by the decomposition of the gas adhere to the inside of the pipings and, as a result, cause the corrosion and the like.
The decomposition of the gas is caused by catalysis of the metal components of the inner wall of the pipings. FIG. 3 illustrates, as an example, the relationship between the temperatures and the decomposition of various gases in the case of Spron. As apparent from FIG. 3, the gases which are 100 ppm at the room temperature, starts decreasing due to the decomposition along with the rise of the temperature.
In the system for the vacuum exhaustion of the process chamber, a direct touch type metal diaphragm valve is commonly used as, for example, shown in the Patent Literature 1.
Basically, the said diaphragm valve comprises a body provided with a flow-in passage, a flow-out passage, and a valve seat formed between the passages; a diaphragm installed in the body and permitted to rest on and move away from the valve seat; and a driving means installed in the body operating to allow the diaphragm to rest on and move away from the valve seat.
[Patent Literature 1] Japanese Patent No. 3343313